The ever-increasing global demand for energy has led to depletion of fossil fuels, which are buried combustible geologic deposits of organic materials that have been converted to crude oil, coal, natural gas, or heavy oils. Because fossil fuels are formed by exposure to heat and pressure in the earth's crust over hundreds of millions of years, they are a finite, non-renewable resource. Further, the burning of fossil fuels is thought to play a key role in global warming. Accordingly, there is a need for non-fossil fuel energy sources.
Hydrocarbons from biological sources represent a cleaner, sustainable alternative energy source. Further, many industries, including plastics and chemical manufacturers, rely heavily on the availability of hydrocarbons for manufacturing processes. Currently, energy-rich lipids and fatty acids (“nature's petroleum”) are isolated from plant and animal oils to produce diverse products such as fuels and oleochemicals. Recent efforts have focused on the microbial production of fatty acids and fatty acid derivatives by cost-effective bioprocesses. Methods of producing fatty acids and/or fatty acid derivatives in microbial hosts are described in, e.g., PCT Publication Nos. WO 2007/136762, WO 2008/119082, WO 2009/009391, WO 2009/076559, WO 2009/111513, WO 2010/006312, WO 2010/044960, WO 2010/118410, WO 2010/126891, WO 2011/008535 and WO 2011/019858 and in Schirmer et al., Science 329(5991):559-562 (2010).
Free fatty acids are known to cause damage to cellular membranes and are thus difficult to produce in amounts sufficient for large scale production. The reduction of fatty acids to more neutral lipids such as wax esters may help to circumvent free fatty acid toxicity. Wax esters possess high energy density relative to shorter-chain biofuel products such as ethanol, and can be produced in cultured cells via a series of enzymatic processes. Wax esters have numerous commercial applications in, e.g., the medical, cosmetic and dietetic industries. For example, wax esters may be used to produce candles, cosmetics, lubricants, printing inks, solvents and fuels.